Semi-crystalline polyolefins can have more than one kind of crystal structure. For example, isotactic polypropylene is known to crystallize into at least three different forms: alpha (monoclinic), beta (pseudohexangonal), and gamma (triclinic) forms. In melt-crystallized material the predominant form is the alpha or monoclinic form. The beta form generally occurs at levels of only a few percent unless certain heterogeneous nuclei are present or the crystallization has occurred in a temperature gradient or in the presence of shearing forces. The heterogeneous nuclei are typically known as beta-nucleating agents, which act as foreign bodies in a crystallizable polymer melt. When the polymer cools below its crystallization temperature, the loose coiled polymer chains orient themselves around the beta-nucleating agent to form beta-phase regions. The beta form of polypropylene is a meta-stable form, which can be converted to the more stable alpha form by thermal treatment and/or applying stress. It is known that micropores can be formed in various amounts when the beta-form of polypropylene is stretched under certain conditions. See, e.g., Chu et al., “Microvoid formation process during the plastic deformation of β-form polypropylene”, Polymer, Vol. 35, No. 16, pp. 3442-3448, 1994, and Chu et al., “Crystal transformation and micropore formation during uniaxial drawing of β-form polypropylene film”, Polymer, Vol. 36, No. 13, pp. 2523-2530, 1995.
In other technologies, during the manufacturing of an article it is sometimes desirable to monitor the presence or position of one or more components to be incorporated into the article. For instance, in a largely automated process for manufacturing disposable absorbent products such as diapers and other incontinence products, certain components (e.g., support layers, absorbent pads, elastic components, and fastener components) must be positioned or aligned with respect to each other and/or other components in order to produce an acceptable product. Inspection systems are commonly used to detect the positions of such components during manufacturing. Some exemplary registration inspection systems such as that disclosed in U.S. Pat. No. 5,359,525 (Weyenberg) employ conventional video cameras for capturing visible and ultraviolet light reflected by and/or transmitted through components in order to produce still video images of such components. After producing a video image of an article and its several components, the image can be analyzed to determine whether the components are properly positioned and registered with one another. Another inspection system described in U.S. Pat. No. 6,927,857 (Koele et al.) utilizes infrared detectors to produce an image from a composite article irradiated with light. One or more of the components of the composite article includes an infrared blocker to enhance the degree to which the component can be detected.